 Two schematic representations of a phospholipid. A phospholipid is a lipid that is a primary component of cell membranes.
Most phospholipids consist of a three-carbon alcohol, glycerol (propan-1,2,3-triol), two fatty acids usually between 14 and 24 carbon atoms in length, a negatively charged phosphate group and often another alcohol attached to the phosphate. The two fatty acids are attached to carbon atoms 1 and 2 of the glycerol, bonded by an ester linkage formed between the carboxyl groups of the fatty acids and the hydroxyl groups of the glycerol, while attached to the third carbon atom with a phosphoester bond is the phosphate group. The resulting compound is called phosphatidate (Diacylglycerol-3-phosphate), and is the simplest phospholipid.
It is, however, uncommon in cell membranes, and is an intermediate compound in the formation of other more complex phospholipids. These are formed by the addition of an alcohol to the phosphate group by creation of a phosphoester bond between the hydroxyl group of the alcohol and the phosphate group. Common alcohol groups added are serine, ethanolamine, choline, glycerol, and inositol, to form phosphatidyl serine, phosphatidyl ethanolamine, phosphatidyl choline, phosphatidyl inositol and diphosphatidyl glycerol (cardiolipin) respectively.
Phosphatidyl choline is the major component of lecithin, and phosphatidyl ethanolamine that of cephalin.
There are other phospholipids that do not have a glycerol backbone, such as sphingomyelin, whose backbone is an amino alcohol called sphingosine. Here, a fatty acid is linked to the amino group by an amide bond, and the phosphate to the primary OH group at the opposite end of the long hydrocarbon chain. The phosphate group also has a choline group attached to it. There are similar lipids that also use sphingosine, but instead of a phosphatidyl choline group, the primary OH group has a sugar group attached it it, forming a glycolipid. Some complex glycolipids, such as gangliosides, may have as many as seven sugar groups attached to them.
Due to its polar nature, the head of a phospholipid is attracted to water (it is hydrophilic). The nonpolar head is not attracted to water and is said to be hydrophobic. When placed in water, phospholipids form a bilayer, where the hydrophobic tails line up against each other, and forms a membrane with hydrophilic heads on both sides extending out into the water. This allows it spontaneously to form liposomes, or small lipid vesicles, which can then be used to transport materials into living organisms and study diffusion rates into or out of a cell membrane. This membrane is partially permeable, very flexible, and has fluid properties, in which embedded proteins and phospholipid molecules are constantly moving laterally across the membrane because of the forces generated by their vibrations. Such movement can be described by the Fluid Mosaic Model, which basically describes the membrane as a "mosaic" of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are basically free to diffuse laterally through the lipid matrix and migrate over the membrane.
- See also: biochemistry, lipid, lipid bilayer, antiphospholipid syndrome, cell membrane;
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